The susceptibility of various metals to corrosion has been extensively studied. One field where this is particularly important is the aircraft or airline industry. The exterior of most aircraft are made primarily of metal material, particularly aluminum and titanium. In order to improve the corrosion resistance of metal component parts, particularly, an exterior surface of metal component parts, conversion coatings have been developed. Conversion coatings are generally electrolytic or chemical films that promote adhesion between the metal and adhesive resins. A common electrolytic process is anodization in which a metal material is placed in an immersing solution to form a porous, micro rough surface into which an adhesive can penetrate. Chemical films for treating titanium or aluminum include phosphate-fluoride coating films for titanium and chromate conversion films for aluminum.
Painting of metal surfaces is also of important commercial interest. In the aircraft or airline industry, the exterior metal surface of many commercial and government aircraft are painted at considerable expense. Techniques have been developed, through the use, for example, conversion coatings or sol gel processes to improve the adhesion of paints, particularly, urethane coatings that are common in the aircraft applications. With respect to sol gel coatings, U.S. Pat. Nos. 5,789,085; 5,814,137; 5,849,110; 5,866,652; 5,869,140; 5,869,141; and 5,939,197 describe sol gel technologies, particularly zirconium-based sol gel technologies for treating metal surfaces to improve corrosion resistance and adhesion, particularly, paint adhesion.
With respect to metal panels that make up an aircraft, sol gel coatings such as those described in the above-referenced patents have been shown to improve adhesion of epoxy-based and polyurethane paints.
Most panels (e.g., metal panels) that make up, for example, the body of an aircraft are held together by fasteners, particularly rivets. Such fasteners, particularly, the exposed surface of such fasteners must meet corrosion resistance standards mandated by aircraft manufacturers. The fasteners must also be able to maintain a coating, such as a paint (e.g., epoxy-based, polyurethane, polyimide) that may be utilized on the panels that make up the aircraft. One problem that has been identified is that paint that otherwise adheres acceptably to the exterior surfaces of aircraft panels, does not acceptably adhere to the fasteners (e.g., rivets) that join the panels. The condition where paint adherence failure occurs with fasteners in the aircraft industry is known as rivet rash.
In addition to paint adherence, metal panels in the aircraft or airline industry must meet certain corrosion resistance standards. One corrosion resistance standard for conversion coatings of aluminum is a salt spray test in accordance with MIL-C-5441. According to this standard, the chemical conversion coated panels undergo salt spray exposure for a minimum of 168 hours and must show no indication of corrosion under examination of approximately 10× magnification. Although not specifically stated in the MIL-C-5541 standard, aircraft manufacturers often require that fasteners such as rivets meet certain corrosion resistance standards. One aircraft manufacturer standard for rivets is a salt spray exposure for a minimum of 48 hours without indication of corrosion.